Discrimination of sulfate from sulfide in carbonates by electron probe microanalysis

Abstract

Using standards and samples previously analyzed by X-ray Absorption Spectroscopy (XAS), and additional materials, we have demonstrated that the valence of sulfur in a carbonate thin section can be speciated reasonably well with an electron microprobe. The wavelength of sulfur K-α radiation varies slightly and systematically with valence; this can be detected as a shift in the peak position (Bragg angle of the crystal) in the wavelength dispersive spectrometer (WDS). In a series of model compounds, sulfate (S6+) and sulfite (S4+) were easily distinguished from sulfur (S80), disulfide (S22−), and sulfide (S2−). Further distinction within these two groups depends on the S concentration and instrument sensitivity; beam damage to the sample is a limiting factor. Microprobe analysis of carbonate samples previously studied by XAS yielded S speciation in the sulfate-sulfite grouping. This is consistent with the earlier, high-resolution, XAS finding of sulfate substituting for carbonate. Speciation of sulfur using the microprobe permits spatially resolved analysis of the S intimately associated with the carbonate material. The technique is extending our assessment of the form of S in a variety of marine bio-carbonates and reservoir dolomites. Determination of sulfur valence also constrains Eh conditions in the solution at the time of mineral formation or diagenesis.